The long-term objective of this study is to develop a therapy for Retinitis Pigmentosa (RP), a disease that causes photoreceptor degeneration and blindness. There is currently no therapy available for this disease. RP is caused by mutations in genes expressed exclusively in the rod photoreceptors. The most common causes of recessive RP are mutations in the gene encoding the subunit of cGMP Phosphodiesterase ( PDE). Despite the availability of a variety of animal models with mutations in PDE, e.g. rd1, rd10, rcd etc., no evidence yet exists for long-term rescue of rod photoreceptors in these models, a prerequisite to clinical trials. This is in stark contrast to the success gene therapy approaches have had in other ocular tissues such as the retinal pigment epithelium (RPE) and specifically in animal models of Leber's congenital amaurosis. The lag in progress for rescue of rod photoreceptor diseases is due to the absence of efficacious gene transfer vectors for photoreceptors. The most commonly used vector in ocular gene therapy to date is adeno-associated virus (AAV), most serotypes of which have a limited cloning capacity of 4.8 Kb. This capacity is insufficient for the inclusion of large gene regulatory elements needed to achieve rod-specific and regulated transgene expression. Furthermore, AAV has been recently shown to cause hepatocellular carcinoma at the alarmingly high rate of 56% in animals by insertional mutagenesis. Hence, there is a need for gene therapy vectors that persist episomally. These significant deficiencies with AAV vectors and other deficiencies discussed in the proposal can be overcome through the use of helper-dependent adenovirus vectors (Hd-Ad) that have a 36K cloning capacity and persist episomally for years in non human primates. However, Hd-Ad vectors do not have a tropism for photoreceptors but instead, they target only the RPE. Recently, we have shown that adenovirus (Ad) vectors with deletions in the RGD domain of penton base or Ad vectors that display transferrin on their capsid through can very efficiently transduce photoreceptors. These modifications should readily be transferable to Hd-Ads. Hence, this proposal has 3 specific aims. Specific Aim 1: Determine whether in the context of 1st generation Ad vectors, do Ad capsids containing an RGD deletion in penton base combined with transferrin on their capsid, transduce photoreceptors more efficiently than either modification alone. Specific Aim 2: To develop a 36Kb-capacity Hd-Ad that has been modified to contain an RGD-deletion in it's penton base and transferrin on it's capsid. Examine rod specific transgene expression using very large (>10Kb) 5' upstream and downstream gene regulatory elements, including a 10Kb PDE promoter. Specific Aim 3: To examine whether the photoreceptor targeted Hd-Ad system can express PDE in the rd1 and rd10 murine retina specifically in rod photoreceptor cells using native (10Kb) PDE 5' and 3' gene regulatory elements and potentiate long term rescue of photoreceptor degeneration. PUBLIC HEALTH RELEVANCE: According to public opinion polls, blindness is second only to cancer as the most feared disease amongst Americans. The most common cause of genetic blindness is Retinitis Pigmentosa, for which there is currently no FDA-approved therapy available. This proposal aims to develop a genetic therapy for Retinitis Pigmentosa and allied disorders that cause blindness. The molecular tools developed in this study will also have application in developing therapies for diabetic retinopathy, age related macular degeneration, glaucoma etc., that are collectively the most common causes of blindness in the United States.